Optical imaging systems and methods which provide real-time detection, diagnosis and imaging of diseases are known in the art. The application of such systems in vivo during real time medical procedures has been limited by a poor signal to noise ratio. The low signal to noise ratio is a consequence of the low strength or absence of an optical signal coming from the target tissue and a high level of background noise from any ambient light. In order to improve the signal to noise the majority of known optical diagnostic systems and methods block the ambient light, ignore it, background subtracting it or turn off room light during measurements. However, turning off the lights or obscuring a view of the patient during a medical procedure can be disadvantageous and possibly dangerous. On the other hand by simply ignoring or background subtracting ambient light, one risk to overflow the detector and/or bury the weak signals of a tissue in the background noise unless the ambient light is specifically rejected. Some other known system and method for rejecting ambient light include gating the detector or modulate the illumination signal and using lock-in detection. However, some of these methods are not applicable to measure weak Raman signals that require continuous integration of the signal for one to several seconds or minutes, in some instances.
Some spectroscopy examination procedures such as for example, Raman spectral measurements or fluorescence spectroscopy can be very sensitive to contamination from the ambient light because Raman or fluorescence signals can be extremely weak and even a small leak of ambient light can impede with the signal. Raman spectroscopy is a spectroscopic technique that operates on the principle that light of a single wavelength striking a molecule is scattered by the molecule through a molecular vibration state transition. The resultant scattered light has wavelengths different than the incident or excitation light. The wavelengths present in the scattered light are characteristic of the structure of the molecule. The intensity and wavelength or “Raman Shift” of the scattered light is representative of the concentration of the molecules in the sample. So, the spectrum of the inelastically scattered radiation represents a fingerprint of the molecular vibrations within the observed sample. Traditionally, the excitation light source, typically a laser, is directed continuously against a target tissue, and the Raman signal is collected over time. In addition to the fact that the Raman signal is naturally very weak, a further problem is the interference with the fluorescence signal due to tissue fluorescence, or emission of light. Many compounds fluoresce or emit light when exposed to laser light in the visible region. Fluorescence bands are generally broad and featureless, and the Raman signal can be often obscured by the fluorescence.
Therefore, there is a need for a system with shaped ambient illumination so that Raman measurements can be carried out under such ambient illumination.